.beta.-amylase is the name traditionally given to exacting maltogenic amylases which catalyze the hydrolysis of 1,4-.alpha.-glucosidic linkages in amylose, amylopectin and related glucose polymers Maltose units are successively removed from the non-reducing chain ends in a stepwise manner until the molecule is degraded or, in the case of amylopectin, until a branch point is approached. The maltose released has the .beta. anomeric configuration hence the name .beta.-amylase.
The .beta.-form of maltose will in aqueous solutions, isomerize spontaneously to a mixture of the .alpha.- and .beta.-forms. Prolonged reaction of .beta.-amylase on amylopectin or partially degraded amylopectin, results in the formation of .beta.-limit dextrin i.e. material which is not susceptible to further hydrolysis by .beta.-amylase.
.beta.-amylases may be used to produce maltose containing syrups of use in the confectionary, baking, and brewing industries.
In the production of high maltose syrups .beta.-amylases are conventionally used in combination with debranching enzymes in order to increase the maltose content in the product by hydrolyzing the 1,6-.alpha.-glucosidic bonds in the .beta.-limit dextrins.
.beta.-amylases have been isolated from various plants and microorganisms (W. M. Fogarty and C. T. Kelly, Progress in Industrial Microbiology, vol. 15, p. 112-115, 1979). These .beta.-amylases are characterized by having optimum temperatures in the range from 40.degree. C. to 65.degree. C. and optimum pH in the range from 4.5 to 7.
Debranching enzymes exhibit for the most a pH optimum in the range from 3.5 to 6 and are thus more active in an acid solution. In the use of .beta.-amylases and debranching enzymes in combination the pH chosen is usually a compromise between the pH optimum of each of the chosen enzymes or the enzymes are used in succession.
In a process for the production of maltose wherein starch in an aqueous solution is hydrolyzed by a .beta.-amylase it is also an advantage to use temperatures at or above 60.degree. C. in order to inhibit retrogradation and to avoid microbial infections.
The vegetal .beta.-amylases further have the disadvantage that their production require large amounts of raw material and a large energy consumption for the extraction and processing, whereas microbial .beta.-amylases can be produced on a large scale at relatively low costs.
One maltogenic amylase has been disclosed in U.S. patent application Ser. No. 591,460, filed on Mar. 3, 1984, now U.S. Pat. No. 4,604,355, which amylase is sufficiently thermostable to be used at temperatures in the range from 60.degree. C. to 65.degree. C. However, this enzyme has its pH optimum at pH 4.5 to 5.5. Also, the enzyme according to said U.S. patent application hydrolyzes maltotriose, which results in the production of glucose.
In high maltose syrups the presence of glucose should be avoided, and the use of said maltogenic amylase consequently requires subsequent removal of the glucose produced to a satisfactory level.
Therefore, there still exists a need for an effective microbial .beta.-amylase preparation which is sufficiently thermostable to be employed at 60.degree.-70.degree. C. for extended periods of time to allow hydrolysis of the starch in an economical way, is sufficiently stable at pH values below 5 to allow for simultaneous hydrolyzation of starch by the .beta.-amylase and a debranching enzyme under optimal conditions, and which in the production of high maltose syrups does not produce excessive amounts of glucose, which in some cases subsequently must be separated from the syrup.
It is an object of the present invention to furnish a novel microbial .beta.-amylase which has a high temperature stability, a pH optimum at pH values below 5 and which produce minimal amounts of glucose.
The present invention is based upon the discovery that a novel microbial extracellular .beta.-amylase (21-51 .beta.-amylase) having such properties is produced from a newly isolated Bacillus strain NCIB 11608.